Theoretical investigation of the structural, electronic and optical properties of ZnO crystals doped with boron at different ratio

In this study, how the doping of ZnO crystal with boron changes the structural, electronic and optical properties was investigated by using density functional theory (DFT) based calculations. Standard DFT cannot fully identify highly correlated d electrons. In order to relatively correct this situation, the DFT+U method was used in the calculations. The location of the energy levels where Zn 3d electrons are concentrated below the valence band is one of the defining features for ZnO. In calculations using standard DFT, the location of these energy levels is calculated incorrectly. This situation can be corrected with the DFT+U method. When the U value was determined as 7.5 eV for 3d electrons in the DFT+U method, it was observed that more compatible results with the experimental data could be obtained. This result is consistent with previous studies. This method was used in all of the calculations in this study. In this study, ZnO crystal was doped with boron atom at a rate of 2.78%, 5.56% and 8.34%. In the optimization calculations, it has been determined that the boron atom causes local distortions around the same in all doping ratios. These local disturbances do not disturb the general structure of the crystal. While the lattice vectors shorten by about 0.1 Å, the angle between them does not change. Boron atoms do not tend to cluster in the crystal structure. The spacing of the valence band maximum and conduction band minimum at three different doping ratios does not change significantly compared to the pure case. The approximate value of this gap in the pure and doped cases is 1.8 eV. The doped boron atom in the crystal prefers to be replaced by the zinc atom. This replacement causes an excess of electrons to form in the ZnO crystal. Since the excess electrons fill the lower levels of the conduction band minimum, the Fermi energy level is also formed here. As the amount of boron in the crystal increases, the Fermi energy level starts from the conduction band minimum and rises to higher energy levels. While the electronic structures of the doping ratios of 2.78% and 5.56% are similar to each other, a different empty energy level is formed above the fermi energy level in the conduction band at a rate of 8.34%. This shows that the doping ratio is at critical levels. This effect is also seen in optical properties. When examined in terms of optical properties, the absorption-energy graphs show that the optical band gap widens at doping rates of 2.78% and 5.56%. However, when the doping ratio is increased to 8.34%, the optical band gap narrows. Accordingly, a blue shift was observed at 2.78% and 5.56% doping rates, while a red shift was observed at 8.34% doping rates.